ad hominem

It's a kind of logical fallacy, which typically involves attacking the opponent on some personal footing that doesn't have to do with his actual arguments.

Google is your friend.

Example of Ad Hominem

1. Bill: "I believe that abortion is morally wrong."
   
   Dave: "Of course you would say that, you're a priest."
   
   Bill: "What about the arguments I gave to support my position?"
   
   Dave: "Those don't count. Like I said, you're a priest, so you have to say that abortion is wrong. Further, you are just a lackey to the Pope, so I can't believe what you say."

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Heh--I loved my logic classes, September. I might have been the only Art student to take Logic, Advanced Logic, then Symbolic Logic. The prof was a bit baffled, but foolishly continued to encourage my interest...